As the permanent submarine support structures for subsea wells, pipeline manifolds and pipeline terminations, the mudmat foundation works under obviously complex, nonlinear and uncertain loading, which makes a great challenge for the research of the stability of mudmat foundation. In this paper, finite element method is utilised to investigate the behaviour of square mudmat foundation without embedment in NC clay to fully 3D loading conditions: V-H-M and V-H-T. According to the analysis results, the relevant failure envelop surfaces and formulas are proposed to predict the capacity of mudmat foundation. These results could be treated as reference for the design work of the mudmat foundation.

Life-extension assessment and reinforcement of in-service monopile foundations are becoming increasingly important as early offshore wind farms approach the middle and late stages of their design lives. To improve the lateral bearing performance of existing monopiles without dismantling the original foundation, this study proposes a cable-reinforced composite system composed of a main monopile, auxiliary piles, and steel cables. A three-dimensional finite element model of a 3.0 MW offshore wind turbine monopile foundation in homogeneous clay was developed using ABAQUS, and the influences of load application angle, auxiliary pile diameter, auxiliary pile length, cable-to-vertical angle, and auxiliary pile number were investigated by a control-variable method. The unreinforced monopile has a baseline lateral bearing capacity of 11.88 MN at a mudline displacement of 0.1D, where D is the monopile diameter. For the three-auxiliary-pile configuration with a 1.5 m auxiliary pile diameter, a 15 m auxiliary pile length, and a 45° cable-to-vertical angle, the capacity reaches 13.65–14.14 MN under different load directions, corresponding to an improvement of 14.91%–19.08%. Increasing the auxiliary pile diameter from 1.0 to 2.0 m raises the capacity under the unfavorable 30° load direction from 13.19 to 14.17 MN, while increasing the auxiliary pile length from 10 to 25 m improves the capacity from 13.11 to 14.94 MN. Increasing the cable-to-vertical angle from 30° to 60° further increases the capacity from 12.92 to 14.66 MN. The four-auxiliary-pile system provides the highest reinforcement efficiency, with a bearing capacity of 14.85–15.61 MN and an improvement ratio of 25.06%–31.41%. In all cases, the maximum cable stress is below 251.4 MPa, far lower than the allowable limit of 1200 MPa. These results indicate that the proposed cable-auxiliary-pile system can effectively enhance the lateral resistance and service-life extension potential of offshore wind monopile foundations.

Helical anchors represent a promising solution for anchor systems of floating offshore wind turbines, yet scour-induced changes to the seabed threaten anchor stability. The bearing capacity and failure mechanisms of helical anchors under such conditions have not been fully investigated. Finite element (FE) analyses examine scour geometry, anchor length, and soil heterogeneity (κ = kD/s_um, where k is the undrained strength gradient, D is the shaft diameter, and s_um is the surface strength). The results demonstrate that scour depth is the dominant factor governing bearing capacity, whereas scour width and slope angle have limited effects. Horizontal capacity is particularly sensitive to scour because the corresponding failure mechanism develops mainly within shallow soil. Increasing κ mitigates scour-induced capacity loss, with this effect being most pronounced as κ rises from 0 to 2, beyond which further increases yield diminishing benefit. Across different helical anchor embedment lengths, scour is shown to shift the critical embedment depth separating shallow and deep failure mechanisms, particularly under low soil heterogeneity. The effects of loading direction are further evaluated within the V-H loading space. Finally, predictive expressions are developed to capture the coupled effects of scour geometry, soil heterogeneity, and anchor length for engineering capacity assessment.

Static pile ended with cone apex angle is widely used in practical engineering. However, the corresponding penetration resistance is still impossible to be predicted accurately. To address this, the penetration process of static pile is simulated by using the coupled Eulerian-Lagrangian (CEL) method in this study. During the penetration process of static pile in clay, the effect of cone apex angle and soil rigidity index on pile-end resistance and the distribution of the plastic zone of the soil around the pile-end are analyzed. The corresponding results indicate that the pile-end resistance is positively correlated with the size of cone apex angle and soil rigidity index, and the pile-end resistance rises with the increase of penetration depth until reaching a certain depth. Besides, the expansion process of the plastic zone near the pile-end is characterized by the expansion of spherical cavity. Since the position of expansion cavity center is different from the traditional theory, the corresponding radius in the traditional theory of cavity expansion is modified according to the corresponding numerical results. Finally, a semi-empirical method, in which the effect of cone apex angle and the soil rigidity index are considered, is proposed to predict the ultimate pile-end resistance of static pile.